1. Field of the Invention
The present application relates to optical metrology, and, more particularly, to modeling and measuring structures with spatially varying properties in optical metrology.
2. Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine various characteristics, such as the profile of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a periodic grating in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated.
Conventional optical metrology is used to determine the deterministic profile of a structure formed on a semiconductor wafer. For example, conventional optical metrology is used to determine the critical dimension of a structure. However, the structure may be formed with a region with a spatially varying property, such as material refractive indices (n and k values), thickness, roughness, and the like. The spatially varying property of the structure is not accounted for in conventional optical metrology, which can produce inaccurate or erroneous results.